Need some input -> Problem with 3.2 transplant going lean and misfiring after two min

[Wayne 962]

Wait a sec. So, that's only one part of the equation. So, you're saying that the current feeding the injectors might not be beefy enough, and that comes from pin 30 (connecting to the battery), that might indeed be the case, but then we'd see a voltage drop (maybe) at the breakout box for the DME, and perhaps voltage drop problems with the ignition system (which we may be seeing). Indeed, I will beef up that wire, and run a separate one to the battery.

But, the injectors are driven off of pin 87 (lines / junction A & B in the diagram above). So, I think you're actually referring to the two wires that say 2.5 RD and head to points A/B. If these are not beefy enough, then this will show up as a problem. This would be the wires from 87 to the six-prong connector (which only has four prongs populated) on the injector harness. I understand now.

So, you want to make sure that the pin in the four pronged connector (which has 12V on it from pin 87 of the DME relay) has enough current to drive the injectors. So, I can run a new wire to pin 30 for the DME relay, and that would assure good current to the ECU, etc. but I should also run a wire from the battery to the two terminals on the four pronged connector (two of which will have 12V when the DME relay is energized.

I get it now.




This connector:



Thx,

Wayne

[Wayne 962]

Originally Posted by wjdunham

You might want to take some contact cleaner to the injector harness connector pins or rough them up with some scotch brite.

Yup, that's a good idea, and I already did that a few days ago. It improved the resistance of the harness...

-Wayne

[Wayne 962]

Originally Posted by ischmitz

And don’t assume whoever did the conversion followed the stock wiring diagram. Maybe they ran power to the coil and/or injectors directly through the ignition switch. You never know.

Indeed, there are these mystery fuses located here that I have not messed with yet. Now is probably the time!





I've never quite liked this particular connection either:




I found this info on the web too that explains it a bit better:

http://performancefuelsystems.com/InjectorCompattibilitywithECUs-TechCorner.htm

This electrical issue would also possibly explain why the missing part doesn't seem to be concentrated on any one cylinder in particular, and also why the problem doesn't appear to change or be affected when the rpm increases. If it were a fuel pressure problem, I would have thought that there would be an issue as the rpm increases...

Hmm...

-Wayne




-Wayne

[ischmitz]

You’re getting the concept: think of all six injectors being a 0.5 Ohm load. Even though they are fed through separate harness branches the DME acts like a giant switch to GND via the two output pins. Internally there is one channel that turns on to thr negative pole of the battery. The other side of that load is supplied by T15 (1st stage of the DME relay in a stock setup) going back to the battery positive.

At 12V and 0.5 Ohm you’re running ~24 Ampere through this load. That’s what the wiring needs to support. Any poor connection, e.g. poor GND to the DME or poor +12V to the other side of the injectors will cause issues.

[FrankM_]

Originally Posted by Wayne 962

Why would plugging in three more injectors on the other side of the non-functioning side of the motor affect the performance? I can think of only two things. Either the fuel flow is being affected by the addition of three more injectors (keep in mind, this is idle fuel flow, not a tremendous amount), or there is some type of electrical load or short circuit in the harness that is causing problems when the three (non-sparked) injectors are plugged in.

Assuming here that this bank of injectors is NOT monitored in any way by the O2 sensor? Otherwise these fuel vapours would certainly indicate 'full rich' and the ECU will intervene

[Wayne 962]

Originally Posted by FrankM_

Assuming here that this bank of injectors is NOT monitored in any way by the O2 sensor? Otherwise these fuel vapours would certainly indicate 'full rich' and the ECU will intervene

For the purpose of our testing here, we have the Oxygen sensor unplugged, which puts the Motronic system into "open loop" mode.

Also, specifically to answer your question, the injectors were plugged in on 4-5-6, without spark, and the O2 sensor is on 1-2-3. But - I believe that the O2 sensor will not "sense" fuel vapors - only ignition / burned exhaust. The CO / HC gas analyzer would detect that (but not the O2 sensor). That is at least my understanding of it.

-Wayne

[Wayne 962]

Originally Posted by ischmitz

You’re getting the concept: think of all six injectors being a 0.5 Ohm load. Even though they are fed through separate harness branches the DME acts like a giant switch to GND via the two output pins. Internally there is one channel that turns on to thr negative pole of the battery. The other side of that load is supplied by T15 (1st stage of the DME relay in a stock setup) going back to the battery positive.

At 12V and 0.5 Ohm you’re running ~24 Ampere through this load. That’s what the wiring needs to support. Any poor connection, e.g. poor GND to the DME or poor +12V to the other side of the injectors will cause issues.

Yes - understand completely. My initial hunch was something in this one-off wire harness was not completely correct, but I was focused on internal resistance and work hardening and I didn't consider the current requirements of the low impedance injectors! But the test today where I electrically "loaded" the six injectors on the harness seemed to point to this theory.

I also ordered more connectors - I can wire each 12V line to each injector directly to the battery if need be to perform the "ultimate" test of current flow.

-Wayne

[proporsche]

hmm ..(maybe i overlooked it )..did not know it is home made el.harness.looks like you are in good hands with isSmithz...

Ivan

[creaturecat]

the forum needs a "like" button.
what a knowledge base!

[wjdunham]

Originally Posted by ischmitz

You’re getting the concept: think of all six injectors being a 0.5 Ohm load. Even though they are fed through separate harness branches the DME acts like a giant switch to GND via the two output pins. Internally there is one channel that turns on to thr negative pole of the battery. The other side of that load is supplied by T15 (1st stage of the DME relay in a stock setup) going back to the battery positive.

At 12V and 0.5 Ohm you’re running ~24 Ampere through this load. That’s what the wiring needs to support. Any poor connection, e.g. poor GND to the DME or poor +12V to the other side of the injectors will cause issues.

The thread is so long I can't find if you might have checked the grounds but that's a very good point here as well. All the 3.2 harness grounds are in the engine compartment bolted to the left side of the intake manifold on a 911, but you've got a custom setup there so might want to see where all those ground connections wind up and clean the contacts there as well. The DME is bolted to the chassis but I believe that ground is just for the outer case of the DME unit to shield it from EMI and prevent it from injecting noise into your radio.

I always though it was rather poor design to have such a high current ground going all the way back to the engine compartment, would have put it right next to the DME on a very short wire. The sharp edge on the signal will couple into everything else as they run along the inside of the harness as well.

Bill

[3rd_gear_Ted]

Originally Posted by ischmitz

You’re getting the concept: think of all six injectors being a 0.5 Ohm load. Even though they are fed through separate harness branches the DME acts like a giant switch to GND via the two output pins. Internally there is one channel that turns on to thr negative pole of the battery. The other side of that load is supplied by T15 (1st stage of the DME relay in a stock setup) going back to the battery positive.

At 12V and 0.5 Ohm you’re running ~24 Ampere through this load. That’s what the wiring needs to support. Any poor connection, e.g. poor GND to the DME or poor +12V to the other side of the injectors will cause issues.

24 amps thru the transistor switching to negative
Big time epiphany

[3rd_gear_Ted]

Originally Posted by Wayne 962

Indeed, there are these mystery fuses located here that I have not messed with yet. Now is probably the time!





I've never quite liked this particular connection either:




I found this info on the web too that explains it a bit better:

Injector Compattibility with ECUs - Tech Corner

This electrical issue would also possibly explain why the missing part doesn't seem to be concentrated on any one cylinder in particular, and also why the problem doesn't appear to change or be affected when the rpm increases. If it were a fuel pressure problem, I would have thought that there would be an issue as the rpm increases...

Hmm...

-Wayne




-Wayne

Suggest a nearby mounted terminal board with big ass wire feeding it for all the auxiliary power sources so as to have the same mechanical connection integrity and resistance

[ischmitz]

Originally Posted by 3rd_gear_Ted

24 amps thru the transistor switching to negative
Big time epiphany

It’s not that simple: peak current only happens for less that a millisecond during the opening pulse and 24A is a purely theoretical value assuming zero loss in wiring, the transistor and all connections. It’s to illustrate that even small imperfections will have a big impact on the ability to properly control the injectors.

Transconductance of older BJT Darlington transistors are pretty poor by today’s standards and that’s why Bosch had to employ the P&H topology.

[mysocal911]

Originally Posted by Wayne 962

Why would plugging in three more injectors on the other side of the non-functioning side of the motor affect the performance? I can think of only two things. Either the fuel flow is being affected by the addition of three more injectors (keep in mind, this is idle fuel flow, not a tremendous amount), or there is some type of electrical load or short circuit in the harness that is causing problems when the three (non-sparked) injectors are plugged in. I’m just thinking (especially after observing the injectors in the videos above), that the fuel flow of the injectors is just not enough to make any difference on the system. It’s a miniscule amount of fuel.

So what would cause the engine to start to stumble when the non-spark driven injectors are plugged in?

Remember, the inductive load of the connected injectors affects the injector pulse width, given the design of the
911 3.2 fuel injection system versus the later 964/993 design. As a result, the effective fuel mixture gets changed.

[mysocal911]

Originally Posted by ischmitz

Transconductance of older BJT Darlington transistors are pretty poor by today’s standards and that’s why Bosch had to employ the P&H topology.

You're joking, right? Where did that get pulled from?

[Wayne 962]

Well, I ran the 10 gauge wire last night and just started the car up. No change. BUT - the 12V lead is indeed only one side of the equation. I did check all of my grounds recently for resistance, but not necessarily thinking about current draw. In order to have the large current flow you need to have a supply *and* a return. So, the supply of the 12V to the injectors is only one element - the ground connections that are switched by the DME also need to be well grounded. I can take a few leads from my breakout box and wire them directly to the negative terminal of the battery to ensure that the unit is well grounded. If that doesn't work, then it's on to the next thought...

-Wayne

[ischmitz]

While you're at it also run a heavy gauge wire to the coil's +12V side. And good point on the GND. That is shared between the 6 injectors and the coil as the biggest complex loads. GND comes to the DME via the DME harness on multiple wires. There is lots of internal splicing going on and one big bundle ends up in the engine compartment on a stock 3.2.

Internally all GND pins of the DME are tied together and they are kept seperate from the the DME housing (shield).

Ingo

[mysocal911]

Most who have worked on engines that use an AFM as an input for load, understand that it's likely that the AFM has been re-adjusted
over time from the factory setting, e.g. for more power. This usually results in an over-rich running condition when warm. Knowledgeable techs
when having rough or rich running conditions, typically check the spring tension of the wiper by moving it at the problematic RPM,
and noticing the effect. The typical AFM wiper spring tension setting is set at 2K RPM for a 1.0 - 1.5 CO without the O2 sensor,
with a warm engine.

[mysocal911]

Originally Posted by ischmitz

that’s why Bosch had to employ the P&H topology.

Many who have reverse engineered both the Porsche & BMW Motronic DMEs of the '80s, have learned that Bosch designed Motronic DMEs
for both low & high impedance injectors. The standard BMWs, e.g. 535s, used high impedance injectors. The M3/M5/M6 engines used low
impedance injectors like the 911 3.2, and as a result had basically the same DME ECU design but a different EPROM.

[ischmitz]

what's your point Loren?

Why do YOU think Bosch employed a complex P&H driver design based on a custom IC and many discrete components instead of just picking a modern IGBT, MOSFet or smart lowside switch

And if you'd done your homework you'd know there are simpler Bosch designs that only work for high-impedance injectors from the same time period. I wonder why....

Originally Posted by mysocal911

Many who have reverse engineered both the Porsche & BMW Motronic DMEs of the '80s, have learned that Bosch designed Motronic DMEs
for both low & high impedance injectors. The standard BMWs, e.g. 535s, used high impedance injectors. The M3/M5/M6 engines used low
impedance injectors like the 911 3.2, and as a result had basically the same DME ECU design but a different EPROM.